Methods for producing a log of material properties

ABSTRACT

A method for making a log of material properties in a plurality of beds from an instrument utilizes steps such as estimating material properties for said plurality of beds and/or estimating positions for a plurality of bed boundaries and/or estimating orientations for said plurality of bed boundaries wherein the bed boundary orientations are individually variable. The estimated positions, orientations, and/or material properties can be utilized to compute the log.

This application is a continuation of U.S. Utility patent applicationSer. No. 12/937,311, filed Oct. 11, 2010, which is a national stageentry under 35 USC § 371 of Patent Cooperation Treaty Application No.PCT/US2009/040704, filed Apr. 17, 2008. PCT Application No.PCT/US2009040704 claims benefit of U.S. Provisional Application61/124,594 filed Apr. 17, 2008, U.S. Provisional Application 61/054,881filed May 21, 2008, and U.S. Provisional Application 61/206,584 filedFeb. 2, 2009. Each of the aforementioned patent applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of logging ofmaterial properties and, in one possible specific embodiment, relates tomethods and/or devices for making a log in layered environments. Onepossible non-limiting example includes producing a log of materialproperties with respect to borehole depth.

2. Description of the Background

Subsurface geological formations typically comprise layers of varioustypes of formations. While the present invention is not limited to usein producing logs of a layered environment comprising subsurfacegeological formations, an embodiment of the invention is convenientlydescribed in terms of this environment.

Most oil and gas was originally deposited in an ocean environment. As aconsequence, such formations may contain fluids such as salt waterand/or oil. Salt water, with its mobile sodium and chlorine ions makesthe formation conductive to electricity, while the oil/gas makes theformation resistive. The oil companies typically utilize logging toolsto produce a log of material properties of a wellbore. As one example,when the desired rock formation or depth is reached, the drill pipe andthe bit are removed from the hole. An instrument is lowered into thewellbore to measure the electrical conductivity versus depth. In thisway, a log or a record of the geologic formation is produced. Otherinstruments may generate a log of a wellbore while drilling. Generally,if the rocks are relatively conductive, they contain salt water. If therocks are relatively resistive, they contain oil and/or gas.

The earliest instruments used direct current and were first used in1927. In the 1950's, electromagnetic or induction tools were introduced.These electromagnetic instruments had coaxial coils, and measured justone component of the conductivity tensor of the rock. There are manydifferent electromagnetic tools which measure various physicalquantities. The standard induction tools measure a voltage while themeasurement-while-drilling (MWD) tools measure phase differences and/oramplitude ratios. Other tools comprise many configurations such aslaterolog tools, normal and lateral tools, e-log tools and the like. Thepresent invention may be utilized with these and other tools.

Oil is often deposited in a layered environment. There is an exactmathematical solution to an electromagnetic instrument penetrating aparallel layered environment at any angle as per an article by theinventor. See, for example, Hardman and Shen, “Theory of Induction Sondein Dipping Beds,” Geophysics Vol. 51, No. 3, March 1986, p. 800-809.However, in the real world, the interface between the layers is notnecessarily parallel.

Other background material may include Hardman and Shen, “Charts forCorrecting Effects of Formation Dip and Hole Deviation on InductionLogs,” The Log Analyst, Vol. 28, No. 4, p 349-356, July-August 1987;Hardman, “Four-Term Decomposition Techniques for a Faster Inversion ofInduction Responses,” SPE 84606, October 2003; Wang, Barber, et al.,“Triaxial Induction Logging; Theory, Modeling, Inversion, andInduction,” SPE 103897, December 2006; Anderson, Barbara et al., “Effectof Dipping Beds on the Response of Induction Tools”, SPE FormationEvaluation (March 1988), pp. 29-36; Barber, Anderson, et al,“Determining Formation Resistivity Anisotropy in the Presence ofInvasion, SPE 90526, September 2004; Anderson, Barbara et al., “Responseof 2-MHZ LWD Resistivity and Wireline Induction Tools in Dipping Bedsand Laminated Formations”, SPWLA 31st Annual Logging Symposium, Jun.24-27, 1990, Paper A, pp. 1-25; Barber, Thomas D. et al.,“Interpretation of Multiarray Induction Logs in Invaded Formations atHigh Relative Dip Angles”, The Log Analyst, vol. 40, No. 3 (May-June1999), pp. 202-21; Sommerfeld Partial Differential Equations in Physics,Academic Press 1949; U.S. Pat. No. 3,808,520; U.S. Pat. No. 6,304,086;U.S. Pat. No. 6,573,722; U.S. Pat. No. 6,216,089; U.S. Pat. No.3,510,757; US 2006/0038571; US 2007/0256832; US 2003/0222651; US2003/0146753; US 2003/0155924; US 2005/0127917; US 2004/0017197; US2006/0192562; US 2003/0146751; US 2009/0018775; US 2008/0078580; US2008/0210420; US 2008/0215241; US 2008/0258733; US 2008/0078580; US2008/0278169; and US 2005/0256642.

Since around 2000, the tools have transmitter and receiver coils in thex, y and z directions. These tri-axial instruments measure all thecomponents of the conductivity tensor and are able to orient theindividual bed boundaries. A change in bed boundary orientation may beindicative of a change in the depositional environment. Informationconcerning the orientation of the bed boundary may be very useful in thegeologic interpretation of the formation.

Consequently, there remains a long felt need for improved methods whichmay be utilized to produce more accurate logs in layered environmentswherein the layers may or may not be parallel. Moreover, it sometimesdesirable to more quickly calculate or invert logs. Because thoseskilled in the art have recognized and attempted to solve these problemsin the past, they will appreciate the present invention, which addressesthese and other problems.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved method of loggingphysical properties.

Another possible object of the present invention is to provide a fastermethod of computing and inverting a log.

Another possible object of the present invention is provide the abilityto compute the log when any boundary has an individually variableorientation whereby the bed boundary effects are accurately accountedfor in the log. (FIG. 1)

Another possible object of the invention is to provide improved accuracyof the positions and/or orientations of bed boundaries and/or thematerial properties of the beds.

Another possible object of the invention is to provide an improvedmethod for geosteering a well.

Another possible object of a specific embodiment of the presentinvention is to determine a conductivity log from the composite magneticfield.

These and other objects, features, and advantages of the presentinvention will become apparent from the drawings, the descriptions givenherein, and the appended claims. However, it will be understood that theabove-listed objectives and/or advantages of the invention are intendedonly as an aid in understanding aspects of the invention, are notintended to limit the invention in any way, and therefore do not form acomprehensive or restrictive list of objectives, and/or features, and/oradvantages.

Accordingly, the present invention comprises a method for making a logof material properties in a plurality of beds from an instrument whichproduces an electromagnetic field. (See FIG. 11) In one embodiment, theinvention may comprise steps such as, for example, estimating materialproperties for the plurality of beds and/or estimating positions for aplurality of bed boundaries and/or estimating orientations for theplurality of bed boundaries wherein the bed boundary orientations areindividually variable. Other steps may comprise utilizing the positions,and/or the orientations, and/or the material properties to compute thelog. (See FIG. 1)

In one possible embodiment, an inversion process can be utilized toinvert the log.

In one embodiment, the method may comprise varying the orientation for aselected one of a plurality of bed boundaries and utilizing theresulting orientations of the bed boundaries and/or the positions and/orthe material properties of the beds to compute the log. The method maycomprise varying a position for the selected bed boundary and utilizingthe resulting positions of the bed boundaries and/or the orientationsand/or the material properties to compute the log. The method may alsocomprise varying the material properties for a selected bed andutilizing the resulting material properties and/or positions and/or theorientations of the bed boundaries to compute the log.

In one embodiment, the method may comprise selecting a bed boundary fromthe plurality of bed boundaries, computing a transverse magnetic termone, and utilizing the transverse magnetic term one to compute the log.(See FIG. 6) The method may also comprise computing a transversemagnetic term two and a transverse magnetic term three and/or atransverse magnetic term four for the bed boundary and utilizing thetransverse magnetic term one and/or the transverse magnetic term two andthe transverse magnetic term three and/or the transverse magnetic termfour to compute the log.

In one possible embodiment, the method may comprise selecting a bed fromthe plurality of beds wherein the bed has an upper bed boundary and alower bed boundary (See FIG. 5), computing a first transverse magneticterm one for the upper bed boundary, computing a second transversemagnetic term one for the lower bed boundary, combining the firsttransverse magnetic term one and the second transverse magnetic term oneto provide a combined transverse magnetic term one. (See FIG. 9) Othersteps may comprise utilizing the combined transverse magnetic term oneto compute the log. The method may also comprise computing a transversemagnetic term two for the upper bed boundary, computing a transversemagnetic term three for the lower bed boundary, and utilizing thecombined transverse magnetic term one, the transverse magnetic term two,and the transverse magnetic term three to compute the log. (See FIG. 8)

In another embodiment, the method may comprise selecting a bed boundaryfrom the plurality of bed boundaries, computing a transverse electricterm one, and utilizing the transverse electric term one to compute thelog. The method may comprise computing a transverse electric term twoand a transverse electric term three for the bed boundary and/orutilizing the transverse electric term one and/or the transverseelectric term two and the transverse electric term three to compute thelog.

In another embodiment, the method may comprise selecting a bed from theplurality of beds wherein the bed has an upper bed boundary and a lowerbed boundary, computing a first transverse electric term one for theupper bed boundary, computing a second transverse electric term one forthe lower bed boundary, and combining the first transverse electric termone and the second transverse electric term one to provide a combinedtransverse electric term one. (See FIG. 9) The method may utilize thecombined transverse electric term one to compute the log. The method mayfurther comprise computing a transverse electric term two for the upperbed boundary, computing a transverse electric term three for the lowerbed boundary, and utilizing the combined transverse electric term one,the transverse electric term two, and the transverse electric term threeto compute the log. (See FIG. 8)

In yet another embodiment, the method may comprise selecting a bedboundary from the plurality of bed boundaries, computing a transverseelectric up for the bed boundary and computing a transverse electricdown for the bed boundary. (See FIG. 10) The method may compriseutilizing the transverse electric up and the transverse electric down tocompute the log. The method may also comprise computing a transversemagnetic term one and utilizing the transverse magnetic term one, thetransverse electric up, and the transverse electric down to compute thelog. The method may comprise computing a transverse magnetic term one, atransverse magnetic term two, and a transverse magnetic term three forthe bed boundary, and utilizing the transverse magnetic term one, thetransverse magnetic term two, the transverse magnetic term three, thetransverse electric up, and the transverse electric down to compute thelog.

In another embodiment, the method of claim 1 may comprise selecting abed from the plurality of beds wherein the bed has an upper bed boundaryand a lower bed boundary (See FIG. 10), computing a transverse electricdown for the upper bed boundary and computing a transverse electric upfor the lower bed boundary. The method may utilize the transverseelectric down and the transverse electric up to compute the log. Themethod may also comprise computing a first transverse magnetic term onefor the upper bed boundary, computing a second transverse magnetic termone for the lower bed boundary, combining the first transverse magneticterm one and the second transverse magnetic term one to provide acombined transverse magnetic term one, and utilizing the combinedtransverse magnetic term one, the transverse electric up, and thetransverse electric down to compute the log. The method may alsocomprise computing a first transverse magnetic term one for the upperbed boundary, computing a second transverse magnetic term one for thelower bed boundary, combining the first transverse magnetic term one andthe second transverse magnetic term one to provide a combined transversemagnetic term one, computing a transverse magnetic term two for theupper bed boundary, computing a transverse magnetic term three for thelower bed boundary, and utilizing the combined transverse magnetic termone (See FIG. 9), the transverse magnetic term two, the transversemagnetic term three, the transverse electric up and the transverseelectric down to compute the log. (See FIG. 10)

In another embodiment, the method may comprise computing a first and/ora new transverse electric up for the bed boundary, computing a firstand/or a new transverse electric down for the bed boundary and utilizingthe transverse magnetic term one, the transverse electric up and thetransverse electric down to compute the log.

In another embodiment, the method may comprise computing a first and/ora new transverse electric up for the bed boundary, computing a firstand/or a new transverse electric down for the bed boundary, andutilizing the transverse magnetic term one, the transverse magnetic termtwo, the transverse magnetic term three, the first and/or new transverseelectric up and/or the first and/or new transverse electric down tocompute the log.

In another embodiment, the method may comprise computing a first and/ornew transverse electric down for the upper bed boundary, computing afirst and/or new transverse electric up for the lower bed boundary,utilizing the combined transverse magnetic term one, the first and/ornew transverse electric down and/or the first and/or transverse electricup to compute the log.

In another embodiment, the method may comprise computing a first and/ornew transverse electric down for the upper bed boundary, computing afirst and/or new transverse electric up for the lower bed boundaryand/or utilizing the combined transverse magnetic term one, thetransverse magnetic term two, the transverse magnetic term three, and/orthe first and/or new transverse electric down and the first and/or newtransverse electric up to compute the log.

In yet another embodiment, the method may comprise determining a changein a transverse electric part to estimate bed material propertyderivatives for the plurality of beds (See FIG. 12), and/or estimatingnew material properties for the plurality of beds using the bed materialproperty derivatives for the plurality of beds. If desired, the abovesteps can be iterated until a convergence criteria is reached. A changein a constant related to the material property in the transverseelectric part can be utilized for determining the bed materialderivatives. In one embodiment, the constant can be described utilizingk², where k² is ω²με.

The transverse electric part may comprise an upper bed boundary term anda lower bed boundary term for each of the plurality of beds. Thetransverse electric part can be a transverse electric part of a dipole.The dipole can comprise a vertical dipole component and/or a horizontaldipole component. (See FIG. 7)

The invention provides improved accuracy because the plurality of bedboundaries may or may not comprise non-parallel bed boundaries.

In one embodiment, the method may comprise a method geo-steering theinstrument relative to a first bed boundary. The method may comprisedetermining an orientation between the instrument and the first bedboundary. The method may also comprise determining a relative positionbetween the instrument and the first bed boundary.

The method may comprise estimating apparent dip angles for the pluralityof bed boundaries. Another advantageous feature of the present inventionis that the estimation of the apparent dip angles includes thepossibility that the apparent dip angle calculations include thepossibility the dip angle is changing within the bed rather thanassuming the dip angle is constant within a bed.

In another embodiment, a method for determining a conductivity log maycomprise determining a composite magnetic field at a receiver of aninstrument and generally determining the conductivity log from thecomposite magnetic field. As noted above, the method may comprisedetermining a relative position between the instrument and a bedboundary and/or determining a relative angle between the instrument anda bed boundary.

In another embodiment, a method is provided for making a log of materialproperties in a plurality of beds from an instrument which produces anelectromagnetic field. The method may comprise estimating a firstmaterial property for the plurality of beds and/or estimating a firstorientation and/or a first position for a plurality of bed boundaries.Additional steps may call for utilizing the first orientation and/or thefirst position and/or first material property to compute the log.Additional steps may comprise comparing the so-computed log with ameasured log from the instrument. The method may further compriseiteratively varying at least one of the first material property, thefirst orientation, and/or the first position for at least one of theplurality of bed boundaries and/or beds and subsequently comparing untilthe log is within a convergence criteria of the measured log.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments to the invention, which may be embodied in variousforms. It is to be understood that in some instances various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

FIG. 1 is a simplified elevational view, in cross section, of anenvironment with a wellbore which extends through non-parallel bedboundaries in accord with one possible embodiment of the presentinvention.

FIG. 2 is a simplified elevational view, in cross section, of anenvironment with a wellbore wherein a distance between non-parallel bedboundaries at the wellbore intersections to the bed boundaries is shownin accord with one possible embodiment of the present invention.

FIG. 3 is a simplified elevational view, in cross section, of anenvironment with a wellbore wherein a distance to non-parallel bedboundaries from a position in the wellbore is shown in accord with onepossible embodiment of the present invention.

FIG. 4 is a simplified elevational view, in cross section, wherein theenvironment of FIG. 3 is reconfigured and the combined perpendiculardistance between bed boundaries from the position in the wellbore isshown in accord with one possible embodiment of the present invention.

FIG. 5 is a plot of a term 1, term 2, and term 3 shown with respect todepth relative to upper and lower bed boundaries in accord with onepossible embodiment of the present invention.

FIG. 6 is a plot of a term 1, term 2, and term 3 shown with respect todepth relative to a bed boundary in accord with one possible embodimentof the present invention.

FIG. 7 is a plot of a composite magnetic dipole with a vertical magneticdipole (VMD) component and a horizontal magnetic dipole (HMD) componentin accord with one possible embodiment of the present invention.

FIG. 8 is a plot of a term 1 and term 2 for an upper bed boundary and aterm 1 and term 3 for a lower bed boundary with respect to depth inaccord with one possible embodiment of the present invention.

FIG. 9 is a plot with respect to depth of an adjusted or combined term 1due to a first term 1 for an upper bed boundary and a second term 1 fora lower bed boundary in accord with one possible embodiment of thepresent invention.

FIG. 10 is a plot with respect to depth of a term 1, term 2, andtransverse electric part due to an upper bed boundary and a term 1, term2, and transverse electric part due to a lower bed in accord with onepossible embodiment of the present invention.

FIG. 11 is a schematic which shows an electromagnetic field produced bya transmitter and detected by a receiver as a voltage, which has aproportional relationship to conductivity of a formation in a thick bed.However, the proportional relationship of voltage to conductivitychanges near a bed boundary, which change may be explained as a resultof a transverse electric part, term 2, and term 3 induced near the bedboundary due to the electromagnetic field produced by the transmitterand detected by the receiver.

FIG. 12 is a plot with respect to depth wherein a derivative of atransverse electric part at a bed boundary is utilized to determine anew transverse electric part, which may then be utilized to invert a login accord with one possible embodiment of the present invention.

FIG. 13 is a simplified elevational view of a vertical magnetic dipole(VMD) at a distance z₀ in a bed with bound boundaries at −h and +h inaccord with one possible embodiment of the present invention.

FIG. 14 is a simplified elevational view of a horizontal magnetic dipole(HMD) at a distance z₀ in a bed with bound boundaries at −h and +h inaccord with one possible embodiment of the present invention.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

In approximating the effect of a tool crossing non-parallel bedboundaries, one embodiment of the present invention divides the probleminto a series of the tool crossings of individual bed boundaries whilemaintaining the angle of the tool relative to each bed boundary.Referring now to the drawings and more particularly to FIG. 1, in thisexample, a tool moving through the hole or wellbore enters the middlebed at one angle and leaves at another because the upper and lower bedboundaries are non-parallel. The present invention is also operable ofchanging the angle of the tool within a bed and/or as the toolapproaches or leaves a bed boundary. The effects of a change in angle ofthe tool may typically be more pronounced near the bed boundaries. InFIG. 1, the hole is assumed to be straight, but need not be.

Referring to FIG. 2, when the tool is at the lower bed boundary, orpoint 1, a distance to the upper bed boundary may preferably bedescribed by a line drawn perpendicular to the upper bed boundary. Whenthe tool is at the upper bed boundary, or point 2, the distance to thelower bed boundary may preferably be described as a line drawnperpendicular to the lower bed boundary. In this example, the apparentthickness of the middle bed increases as the tool moves up hole. Thismakes real world sense because the bed thickness is increasing to theright, referring to FIG. 2. In order to incorporate this aspect into onepossible method of the present invention, it is desirable to have thethickness of the middle bed increase as the tool moves up hole, asdiscussed hereinafter.

Referring to FIG. 3, at an intermediate tool position in the middle bed,or point 3, the distance to the upper bed boundary and the distance tothe lower bed boundary may be represented by drawing lines perpendicularto the respective bed boundaries, as indicated.

So when the tool is at point 3, the apparent thickness of the middlebed, as it would appear to the tool with parallel upper and lower bedboundaries, is shown as combination of these distances in FIG. 4.Accordingly, FIG. 4 shows the two bed boundaries as parallel and the twodistances of FIG. 3 are added together, as illustrated. This apparentthickness changes as the tool moves.

A computed log which is related to π_(z2) ^(TMU) of a vertical magneticdipole (VMD) in parallel beds (49) splits into 4 terms using P′ (52)(see SPE 84606 referenced hereinbefore). For convenience, equations mayoften be referenced herein by showing the equation number inparenthesis.

$\begin{matrix}{\mspace{20mu}{{\pi_{z\; 2}^{TMU} = {\pi_{z\; 2}^{{TMU}\; 1} + \pi_{z\; 2}^{{TMU}\; 2} + \pi_{z\; 2}^{{TMU}\; 3} + \pi_{z\; 2}^{{TMU}\; 4}}}\mspace{20mu}{where}}} & {(54)(1)} \\{\pi_{z\; 2}^{{TMU}\; 1} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} + k_{23}} )( {k_{12} + k_{21}} )e^{\xi_{2}{({z_{0} - z})}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & {(55)(2)} \\{\pi_{z\; 2}^{{TMU}\; 2} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} + k_{23}} )( {k_{12} - k_{21}} )e^{\xi_{2}{({{+ {({z_{0} - h})}} + {({z - h})}})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(56)(3)} \\{\pi_{z\; 2}^{{TMU}\; 3} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2\;}}( {k_{32} - k_{23}} )( {k_{12} + k_{21}} )e^{\xi_{2}{({{- {({z_{0} - {({- h})}})}} - {({z - {({- h})}})}})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(57)(4)} \\{\mspace{20mu}{and}} & \; \\{\pi_{z\; 2}^{{TMU}\; 4} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2\;}}( {k_{32} - k_{23}} )( {k_{12} - k_{21}} )e^{- {\xi_{2}{({z_{0} - z})}}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(58)(5)}\end{matrix}$

Terms one (T1) π_(x2) ^(TMU1) and four (T4) π_(x2) ^(TMU4) are constantsbecause the distance (z₀−z) between the source at z₀ and receiver at zis constant since the transmitter-receiver distance (z₀−z) is fixed atthe manufacture of the tool. Terms two (T2) π_(x2) ^(TMU2) and three(T3) π_(x2) ^(TMU3) decay in the downward and upward directionsrespectfully. (See FIG. 5)

When the beds are parallel, it is the perpendicular distance of thetransmitter at z₀ and receiver at z relative to the bed boundary at +hthat determines the T2 response [note+(z₀−h) and +(z−h) in (3)] and thebed boundary at h that determines the T3 response [note−(z₀−(−h)) and−(z−(−h)) in (4)]. T4 because of the factor e^(−4ξ) ² ^(h) is usuallysmall and not shown. As one example, FIG. 13 and FIG. 14 shows a tool ata distance z₀ from the center of a bed, with bed boundaries at +h and−h.

Associated with each bed and each bed boundary are four (4) terms. FIG.5 shows T1, T2, and T3 for a bed. FIG. 6 shows T1, T2, and T3 for a bedboundary. The fourth term may also be utilized but is typically lesssignificant and is not shown. However, it may be utilized if desired.

For use with an electromagnetic logging tool, the transmitter or sourceis a magnetic dipole. As indicated if FIG. 7, this magnetic dipole canbe resolved into a vertical magnetic dipole (VMD) and a horizontalmagnetic dipole (HMD). The VMD and HMD each split into 4 terms. Theresolution is determined by the apparent dip angle of each bed boundaryrelative to the transmitter in the tool.

As indicated in FIG. 8, one possible way to compute a log in nonparallelbeds is to compute the fields due to T3 of a VMD and of a HMD using aTM-TE split method due to the lower bed boundary. The method involveschanging the thickness of the bed as the tool moves at the angle of thetransmitter relative to the lower bed boundary. The method may thencomprise computing the fields due to T2 of a VMD and a HMD for the upperbed boundary as the tool moves. Other steps may comprise changing thethickness of the bed as the tool moves at the angle of the transmitterrelative to the upper bed boundary

Additional steps may comprise computing T1 and T4 due to the lower bedboundary as the tool moves while changing the thickness of the bed atthe angle of the transmitter relative to the lower bed boundary. T1 andT4 may also be computed due to the upper bed boundary as the tool moveschanging the thickness of the bed at the angle of the transmitterrelative to the upper bed boundary.

If the bed is relatively thick, T4 is small and T1 from the upper bedboundary and lower bed boundary is typically the same at the middle ofthe bed. If the bed is relatively thin, T1 and/or T4 may not be the sameat the middle of the bed. When this occurs it is necessary to adjust T1and/or T4 so they are the same at the middle of the bed. One possibleway to adjust T1 and T4 is to take a combination of T1 and T4 as thetool moves. This may be accomplished in many different ways withaveraging techniques and the like. As one simple example, 100% of T1 andT4 may be taken when the tool is at the bottom of the layer. When thetool is at the middle, 50% may be taken from the bottom and 50% from thetop. When the tool is at the top of the bed, 100% may be taken from thetop of the layer.

In FIG. 9, T1 is adjusted or combined so that it provides continuousreadings between the upper bed boundary and the lower bed boundary.After the adjustment is made, the log is computed by summing theapparent dip corrected T3 from the bottom boundary, T2 from the topboundary and the adjusted or combined T1 and/or T4.

In one embodiment of the method for inverting a log, because the bedboundary angular effect is localized at a single bed boundary, the bedboundary angle of a single bed boundary can be varied until the computedlog best matches the measured log. A bed boundary orientation can bechanged by changing T1, T2, T3 and T4 for that bed boundary andrecombining the terms. A bed boundary can be moved by shifting T1, T2,T3 and T4 associated with that bed boundary and recombining the terms.

It will be appreciated that the above method may also be utilized todetect the orientation and position of a tool for use in geosteeringwhereby it is often desirable to remain within a distance and at anorientation with respect to an upper bed boundary. As used herein, bedboundary may refer not only to two layers of rock but also to afluid/fluid interface such as a water/oil interface, gas/liquidinterface, or the like. Other uses may comprise detecting andorientating fractures.

As suggested by FIG. 10, a more accurate although slower method tocompute a log is to compute the transverse electric, (TE) part for a HMDand for a VMD (see equations (99) through (102)) and T3 of thetransverse magnetic (TM) part for a HMD and for a VMD (see equations (4)and (57)) due to the lower bed boundary as the tool moves, changing thethickness of the bed as the tool moves at the angle of the transmitterrelative to the lower bed boundary. Additional steps may comprisecomputing the TE part for a HMD and for a VMD (see equations (76)through (79)) and T2 of the TM part for a HMD and for a VMD (seeequations (3) and (56)) due to the upper bed boundary as the tool moves,changing the thickness of the bed as the tool moves at the angle of thetransmitter relative to the upper bed boundary.

The method may also utilize T1 for a HMI) and a VMD (see equations (2)and (55)) and T4 for a HMD and a VMD of the TM response (see equations(5) and (58)). Accordingly, the method may comprise computing T1 and/orT4 due to the lower bed boundary as the tool moves, changing thethickness of the bed at the angle of the transmitter relative to thelower bed boundary. Additionally, T1 and T4 may be computed due to theupper bed boundary as the tool moves, changing the thickness of the bedat the angle of the transmitter relative to the upper bed boundary.

Similar to the discussion above, if the bed is relatively thick, T4 issmall and T1 from the upper bed boundary and lower bed boundary istypically same at the middle of the bed. If the bed is relatively thin,T1 and T4 might not be the same at the middle of the bed. When thisoccurs it is necessary to adjust T1 and T4 so they are the same at themiddle of the bed. A way to adjust T1 and T4 is to take a combination ofT1 and T4 as the tool moves. A way to do this is to take 100% of T1 andT4 from the bottom T1 and T4 when the tool is at the bottom. When thetool is at the middle take 50% from the bottom and 50% from the top T1and T4. When at the top of the bed take 100% from the top T1 and T4 asbefore. (See FIG. 9) After the adjustment is made, the log may becomputed by summing the apparent dip corrected TE parts form the top andbottom, T3 from the bottom TM response, T2 from the top TM response andthe adjusted T1 and T4 of the TM response.

Since this bed boundary angular effect is localized to a single bedboundary, the bed boundary angle of a single bed boundary can be varieduntil the computed log best matches the measured log. A bed boundaryorientation can be changed by recomputing T1, T2, T3 and T4 of the TMpart and the TE part associated with that bed boundary and recombiningthe parts as above. A bed boundary can be moved by shifting orrecomputing T1, T2, T3, T4 of the TM part and the TE part associatedwith that bed boundary and recombining the parts as above.

Like the previously described method, this method may also be utilizedto detect the orientation and position of a tool for use in geosteeringwhereby it is often desirable to remain within a distance and at anorientation with respect to an upper bed boundary. Other uses maycomprise detecting and orientating fractures.

As suggested by FIG. 11, the induction instrument performs a process onthe geologic formation. The time varying current from the transmitterinduces electromagnetic fields in the geologic formation. The electricfield induces current via J=σE in the formation. This current in turninduces a voltage in the receiver. In some tools, the induced voltage isdirectly related to the conductivity, σ, of the geologic formation. Atthe middle of a thick bed, there is just T1 and this does not vary withangle. However, the relationship between induced voltage andconductivity may change near bed boundaries. When there are bedboundaries, these bed boundaries induce T2, T3 and T4 of the TM responseand the TE parts which occur only near bed boundaries. As shown by theequations and discussion hereinafter, it will be seen that the TE partsare directly related to the difference in the physical propertiesk²=ω²με (20) between the upper and lower beds. The angle of thetool/hole relative to the formation is determined by the bed boundaries.

One possible way to invert logs to obtain the actual material propertiesmay utilize thin bed material derivatives. A thin bed materialderivative is the change or difference in a log when the actual materialproperty

$k^{2} = {{\omega^{2}\mu\; ɛ} = {\omega^{2}{\mu( {ɛ^{\prime} + {i\;\frac{\sigma}{\omega}}} )}}}$from equations (20) and (16), mainly the electrical conductivity σ of asingle thin bed changes. If the thin bed material derivatives for all ofthe beds are known, prior art methods can be utilized to obtain theactual material properties, assuming the bed boundaries are parallel.However, even with this assumption, prior art methods require that eachderivative is a separate log which must be computed separately, which isvery time consuming. The present method is much faster because the thinbed material derivatives are approximated, which reduces the time toinvert a log to obtain the actual material properties based on utilizingk² (see equation 20), as discussed hereinafter.

Accordingly, referring to equations (6) through (9) and/or equations(80) through (83), π_(z) ^(TEU) (for the case of the source in themiddle or second layer π_(z) ^(TEU) for a VMD for the upper (U) bedboundary at +h becomes:

$\begin{matrix}{{\pi_{z\; 1}^{TEU} = {\frac{M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}\xi_{2}H^{+ h}e^{- {\xi_{1}{({z - h})}}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}\mspace{20mu}{\text{---}h}} & {(80)(6)} \\{\pi_{z\; 2}^{TEU} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{- {\xi_{2}{({z - h})}}}} + {( {\xi_{1} - \xi_{3}} )e^{- {\xi_{2}{({z - h + {4h}})}}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(81)(7)} \\{= {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h\;}}e^{{- 2}\xi_{2}h}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z + h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z + h})}}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(82)(8)} \\{\mspace{20mu}{\text{---} - h}} & \; \\{\pi_{z\; 3}^{TEU} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}e^{{- 2}\xi_{2}h}\xi_{1}2\xi_{2}e^{\xi_{3}{({z + h})}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(83)(9)}\end{matrix}$

The symbol - - - - is used in this case to show the bed boundaries.

π₂ ^(TEU) is proportional to the difference in material propertiesk²=ω²με (20) (k₁ ²−k₂ ²) between the upper (equation (1)) and middle(equation (2)) beds. π_(z) ^(TEU) for a HMD is similar.

π_(z) ^(TEL) (equations (10) through (13)) and/or (equations (103)through (106)) for the case of the source in the middle or second layerπ_(z) ^(TEL) for a VMD for the lower (L) bed boundary at −h becomes:

$\begin{matrix}{\pi_{z\; 1}^{TEL} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}e^{{- 2}\xi_{2}h}\xi_{3}2\xi_{2}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(103)(10)} \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEL} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}e^{{- 2}\xi_{2}h}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z - h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z - h})}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\mspace{11mu}\lambda}}}} & {(104)(11)} \\{= {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z + h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z + h - {4h}})}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(105)(12)} \\{\mspace{20mu}{\text{---} - h}} & \; \\{\pi_{z\; 3}^{TEL} = {\frac{- M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}\xi_{2}H^{- h}e^{\xi_{3}{({z + h})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(106)(13)}\end{matrix}$

π_(z) ^(TEL) proportional to the difference in material propertiesk²=ω²με (20) (k₂ ²−k₃ ²) between the middle (equation (2)) and lower(equation (3)) beds. π_(z) ^(TEL) for a horizontal magnetic dipole (HMD)is similar.

For π_(z) ^(TEU) (π_(z1) ^(TEU),π_(z2) ^(TEU),π_(z3) ^(TEU)), (k₁ ²−k₂²) is outside the integral and for π_(z) ^(TEL) (π_(z1) ^(TEL),π_(z2)^(TEL),π_(z3) ^(TEL)), (k₂ ²−k₃ ²) is outside the integral. Since thedifference in k² is a constant, it is outside the integral in both casesIf k₂ ² of a bed changes, the difference in k² of the bed boundary above(k₁ ²−k₂ ²) and below (k₂ ²−k₃ ²) changes. This will change the TE partsfor the Upper (U) part of π_(z) ^(TE), π_(z) ^(TEU) and the Lower (L)part of π_(z) ^(TE),π_(z) ^(TEL).

This difference in k² is useful in inverting logs because it allows thethin bed material derivatives to be approximated. This difference orchange in the TE part approximates a thin bed material derivative, soone method involves computing the TE integrals without the difference ink². The new differences in k² are computed, and the new TE part iscomputed. The old TE part may be subtracted from the new TE parts tocompute the approximate material derivatives. This provides for a veryquick computation of material derivatives.

FIG. 12 shows a visual example of the new TE part and the old TE partfor the upper bed boundary at +h. The method may comprise doing the samething for the lower bed boundary at −h.

Accordingly, the method may use the material derivatives to invert thelog, to obtain the actual material properties

$k^{2} = {{\omega^{2}\mu\; ɛ} = {\omega^{2}{\mu( {ɛ^{\prime} + {i\;\frac{\sigma}{\omega}}} )}}}$(equations (20) (16)).

The convergence criterion used to stop the iteration process varies withthe situation. If a rough estimate is required, a 10 (ten) percentdifference between the measure and computed result might be sufficient.If a better result is required a 1 (one) percent difference might berequired.

Accordingly, one embodiment of the method allows quickly changing a logby changing a constant involving k².

Electromagnetic Fields Due to a Vertical Magnetic Dipole

For sinusoidally time-varying fields with time variation taken ase^(−iwt), Maxwell's equations take the form∇×H=−iωεE  (14)and∇×E=iωμH+iωμM _(s).  (15)It is assumed that the only source in the medium is a magnetic dipolewith dipole moment, M_(s). The complex permittivity ε in equation (14)is:

$\begin{matrix}{ɛ = {ɛ^{\prime} + {i\;\frac{\sigma}{\omega}}}} & (16)\end{matrix}$where ε′ is the dielectric permittivity and σ is the electricalconductivity of the medium.

It can be shown that if a vector potential function, which may be calledthe Hertz vector potential π, is introduced, thenE=iωμ∇×π,  (17)H=∇(∇·π)+k ²π,  (18)and∇² π+k ² π=−M _(s),  (19)wherek ²=ω²με.  (20)

Now consider the vertical magnetic dipole (VMD) shown in FIG. 13. Due tothe rotational symmetry about the z-axis of the geometry, thecylindrical coordinate system is used. The magnetic dipole is located atx=0, y=0, or ρ=0 and z=z₀, and pointing in the z direction, while thebed boundaries are at z=±h.

For a VMD, equation (19) reduces to a scalar equation.∇²π_(z) +k ²π_(z) =−M _(v)δ(r−z ₀ {circumflex over (z)}),  (21)where M_(v) is the vertical component of the total dipole moment M_(s).

The particular solution of equation (21) is π_(z0)

$\begin{matrix}{{\pi_{z\; 0} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\xi}e^{{- \xi}{{z - z_{0}}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}},} & (22)\end{matrix}$where J₀ is the zero order Bessel function of the first kind andξ=(λ² −k ²)^(1/2).  (23)

The branch of ξ is so chosen that ξ=λ as λ approaches infinity and ξ=−ikfor λ=0.

The components of the electromagnetic field are:

$\begin{matrix}{E_{\phi} = {{- i}\;{\omega\mu}\frac{{\delta\pi}_{z}}{\delta\rho}}} & (24)\end{matrix}$from (17) and

$\begin{matrix}{H_{\rho} = {{\frac{d\;}{d\;\rho}\lbrack \frac{d\;\pi_{z}}{d\; z} \rbrack} = {\frac{d\;}{d\; z}\lbrack \frac{d\;\pi_{z}}{d\;\rho} \rbrack}}} & (25)\end{matrix}$from (18).

To satisfy the boundary condition on tangential E, and E_(ϕ), and H,H_(ρ), at z=±h, the following boundary conditions must be satisfied.From equationπ_(zj)=π_(z(j+1)),  (26)and from equation (25):

$\begin{matrix}{\frac{d\;\pi_{zj}}{d\; z} = {\frac{d\;\pi_{z{({j + 1})}}}{d\; z}.}} & (27)\end{matrix}$

The above boundary conditions are enforced at z=h for j=1 and at z=−hfor j=2. π_(z) of a VMD, can be expressed in terms of π_(z) ^(TM) whichis mathematically transverse magnetic (TM) and π_(z) ^(TE) which isphysically transverse electric (TE):π_(z)=π_(z) ^(TM)+π_(z) ^(TE).  (28)

π_(z) ^(TM) is physically TE because the E field using E=iωμ∇×(π_(z)^(TM){circumflex over (z)}) (17) is in the x and y directions, which aretransverse to the z axis. Therefore, (π_(z) ^(TM)+π_(z) ^(TE)) isphysically TE.

Restating the boundary conditions for π_(z) assuming μ₁=μ₂=μ₃=μ:

$\begin{matrix}{\pi_{zj} = \pi_{z{({j + 1})}}} & {(26)(29)} \\{\frac{d\;\pi_{zj}}{d\; z} = {\frac{d\;\pi_{z{({j + 1})}}}{d\; z}.}} & {(27)(30)}\end{matrix}$

Let the equivalent boundary conditions on π_(z) ^(TM) and π_(z) ^(TE)be:

$\begin{matrix}{\frac{d\;\pi_{zj}^{TE}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TE}}{d\; z}} & (31) \\{\frac{d\;\pi_{zj}^{TM}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TM}}{d\; z}} & (32) \\{{\pi_{zj}^{TM} + \pi_{zj}^{TE}} = {\pi_{z{({j + 1})}}^{TM} + \pi_{z{({j + 1})}}^{TE}}} & (33) \\{{x_{j}\pi_{zj}^{TM}} = {x_{({j + 1})}{\pi_{z{({j + 1})}}^{TM}.}}} & (34)\end{matrix}$

Note that equation (33) is equivalent to (26) or (29) using (28).Equation (31) plus (32) is equivalent to (27) or (30) again using (28).x_(j) in (34) can be anything, such as k_(j) ². Setting x_(j) to bek_(j) ² in (34). Thus the equivalent boundary conditions (31) through(34) become:

$\begin{matrix}{\frac{d\;\pi_{zj}^{TE}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TE}}{d\; z}} & {(31)(35)} \\{\frac{d\;\pi_{zj}^{TM}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TM}}{d\; z}} & {(32)(36)} \\{{\pi_{zj}^{TM} + \pi_{zj}^{TE}} = {\pi_{z{({j + 1})}}^{TM} + \pi_{z{({j + 1})}}^{TE}}} & {(33)(37)} \\{{k_{j}^{2}\pi_{zj}^{M}} = {k_{({j + 1})}^{2}{\pi_{z{({j + 1})}}^{TM}.}}} & (38)\end{matrix}$

Restating the boundary conditions, equation (37) is equivalent to (26)or (29) using (28). (35) plus (36) is equivalent to (27) and (30) againusing (28). The above boundary conditions (35) through (38) are enforcedat z=h for j=1 and at z=−h for j=2.

The Hertz potential π_(z) ^(TM) satisfies the inhomogeneous partialdifferential equation (19) with M_(s)=M_(v)δ(r−z₀{circumflex over(z)}){circumflex over (z)}. Accordingly π_(z) ^(TM) can be expressed as:

$\begin{matrix}{\mspace{79mu}{{\pi_{z\; 1}^{TM} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{1}}{\xi_{1}}e^{{- \xi_{1}}{{z - z_{0}}}}} + {P_{1}e^{- {\xi_{1}{({z - h})}}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}\mspace{79mu}{\text{---}\mspace{11mu} h}}} & (39) \\{{\pi_{z\; 2}^{TM} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{2}}{\xi_{2}}e^{{- \xi_{2}}{{z - z_{0}}}}} + {Q_{2}e^{\xi_{2}{({z - h})}}} + {P_{2}e^{- {\xi_{2}{({z + h})}}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}\mspace{79mu}{\text{---}\mspace{11mu} - h}} & (40) \\{\mspace{79mu}{\pi_{z\; 3}^{TM} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{3}}{\xi_{3}}e^{{- \xi_{3}}{{z - z_{0}}}}} + {Q_{3}e^{\xi_{3}{({z + h})}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}} & (41)\end{matrix}$

The symbol - - - - is used in this case to show the bed boundaries at±h, and later to show the position of the source at z₀ (see equationsbetween (49) and (50)). Because the source could be in any of the threelayers, the β s are flags that indicate in which medium the source islocated. For example, if the source is in the middle or second medium,β₂=1 and β₁=β₃=0. By applying the boundary conditions (see equations(36) and (38)), we obtain four equations to solve for the four unknownfunctions P₁, Q₂, P₂ and Q₃.

For the case of the source in the middle or second layer, β₂=1:

$\begin{matrix}{\mspace{79mu}{P_{1} = {{\frac{e^{{- 2}\xi_{2}h}}{\Delta_{h}\xi_{2}}\lbrack {{( {k_{32} + k_{23}} )e^{\xi_{2}{({z_{0} + h})}}} + {( {k_{32} - k_{23}} )e^{\xi_{2}{({z_{0} + h})}}}} \rbrack}2k_{22}}}} & (42) \\{Q_{2} = {\frac{e^{{- 2}\xi_{2}h}}{\Delta_{h}\xi_{2}}{( {k_{12} - k_{21}} )\lbrack {{( {k_{32} + k_{23}} )e^{\xi_{2}{({z_{0} + h})}}} + {( {k_{32} - k_{23}} )e^{- {\xi_{2}{({z_{0} + h})}}}}} \rbrack}}} & (43) \\{{P_{2} = {\frac{e^{{- 2}\xi_{2}h}}{\Delta_{h}\xi_{2}}{( {k_{32} - k_{23}} )\lbrack {{( {k_{12} + k_{21}} )e^{\xi_{2}{({z_{0} - h})}}} + {( {k_{12} - k_{21}} )e^{- {\xi_{2}{({z_{0} - h})}}}}} \rbrack}}}\mspace{70mu}{and}} & (44) \\{\mspace{79mu}{{Q_{3} = {{\frac{e^{{- 2}\xi_{2}h}}{\Delta_{h}\xi_{2}}\lbrack {{( {k_{12} + k_{21}} )e^{- {\xi_{2}{({z_{0} + h})}}}} + {( {k_{12} - k_{21}} )e^{\xi_{2}{({z_{0} - h})}}}} \rbrack}2k_{22}}}\mspace{76mu}{where}}} & (45) \\{\mspace{79mu}{{\Delta_{h} = {{( {k_{12} + k_{21}} )( {k_{32} + k_{23}} )} - {( {k_{12} - k_{21}} )( {k_{32} - k_{23}} )e^{{{- 4}\;\xi_{2}h}\;}}}}\mspace{79mu}{and}}} & (46) \\{\mspace{79mu}{k_{mn} - {k_{m}^{2}{\xi_{n}.}}}} & (47)\end{matrix}$

For the case of the source in the middle or second layer β₂=1, themiddle layer splits into an upper (U) part where z>z₀,π_(z2) ^(TMU), anda lower (L) part where z<z₀, π_(z2) ^(TML):

$\begin{matrix}{\mspace{79mu}{{\pi_{z\; 1}^{TM} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{e^{{- 2}\;\xi_{2}h}}{\Delta_{h}\xi_{2}}P^{\prime}2k_{22}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}\mspace{76mu}{\text{---}h}}} & (48) \\{{\pi_{z\; 2}^{TMU} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{e^{{- 2}\;\xi_{2}h}}{\Delta_{h}\xi_{2}}{P^{\prime}\lbrack {{( {k_{12} + k_{21}} )e^{- {\xi_{2}{({z - h})}}}} + {( {k_{12} - k_{21}} )e^{\xi_{2}{({z - h})}}}} \rbrack}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}}\mspace{70mu}{\text{---}{z_{0}\mspace{14mu}}_{\mspace{50mu}}}} & (49) \\{{{\pi_{z\; 2}^{TML} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{e^{{- 2}\;\xi_{2}h}}{\Delta_{h}\xi_{2}}{Q^{\prime}\lbrack {{( {k_{32} + k_{23}} )e^{\xi_{2}{({z + h})}}} + {( {k_{32} - k_{23}} )e^{- {\xi_{2}{({z + h})}}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}\mspace{76mu}{\text{---}\; - h}}\mspace{70mu}} & (50) \\{\mspace{79mu}{{\pi_{z\; 3}^{TM} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{e^{{- 2}\xi_{2}h}}{\Delta_{h}\xi_{2}}Q^{\prime}2k_{22}e^{\xi_{3}{({z + h})}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}}\mspace{79mu}{where}}} & (51) \\{\mspace{79mu}{{P^{\prime} = {{( {k_{32} + k_{23}} )e^{\xi_{2}{({z_{0} + h})}}} + {( {k_{32} - k_{23}} )e^{- {\xi_{2}{({z_{0} + h})}}}}}}\mspace{79mu}{and}}} & (52) \\{\mspace{79mu}{Q^{\prime} = {{( {k_{12} + k_{21}} )e^{- {\xi_{2}{({z_{0} - h})}}}} + {( {k_{12} - k_{21}} )e^{\xi_{2}{({z_{0} - h})}}}}}} & (53)\end{matrix}$

Using P′ (52), π_(z2) ^(TMU) (49) splits into four (4) terms. For thecase of the source in the middle or second layer β₂=1:π_(z2) ^(TMU)=π_(z2) ^(TMU1)+π_(z2) ^(TMU2)+π_(z2) ^(TMU3)+π_(z2)^(TMU4)  (1)(54)where

$\begin{matrix}{\pi_{z\; 2}^{{TMU}\; 1} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} + k_{23}} )( {k_{12} + k_{21}} )e^{\xi_{2}{({z_{0} - z})}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & {(2)(55)} \\{\pi_{z\; 2}^{{TMU}\; 2} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} + k_{23}} )( {k_{12} + k_{21}} )e^{\xi_{2}{({{+ {({z_{0} - h})}} + {({z - h})}})}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & {(3)(56)} \\{{\pi_{z\; 2}^{{TMU}\; 3} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} - k_{23}} )( {k_{12} + k_{21}} )e^{\xi_{2}{({{- {({z_{0} - {({- h})}})}} - {({z - {({- h})}})}})}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}}\mspace{79mu}{and}} & {(4)(57)} \\{\pi_{z\; 2}^{TMU4} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{1}{\Delta_{h}\xi_{2}}( {k_{32} - k_{23}} )( {k_{12} - k_{21}} )e^{- {\xi_{2}{({z_{0} - z})}}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & {(5)(58)}\end{matrix}$

The above equations apply when the both the source at z₀ and thereceiver at z are in the middle bed. Four terms are present when thesource and receiver are in different beds, although term one (1) andterm four (4) are not constants.

Similarly π_(x2) ^(TML) (see equation (50)) splits into 4 terms using Q′(see equation (53)).

Of the four terms, term one is the one that is most like a complete log.If only one term were to be computed, it should be term one. It is alsouseful in computing other terms. For example, if term two in MWD wererequired it could be computed by computing a log with terms one and twothen subtracting term one form the log. This is useful in MWD becausephase differences and/or amplitude ratios are measured.

The boundary condition on (π_(z) ^(TM)+π₂ ^(TE)) (37) may be thought ofas a coupling mechanism between π_(z) ^(TM) and π_(z) ^(TE). Thehomogeneous solution of equation (21) having cylindrical symmetry aboutthe z-axis is:

$\begin{matrix}{\pi_{zj}^{TE} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\begin{matrix}{{S_{j}(\lambda)}e^{- {\xi_{j}{({z \pm h})}}}} \\{{T_{j}(\lambda)}e^{+ {\xi_{j}{({z \mp h})}}}}\end{matrix}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & (59)\end{matrix}$

Therefore, π_(z) ^(TE) in each of the three layers for a VMD is:

$\begin{matrix}{{\pi_{z\; 1}^{TE} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{S_{1}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}},{\text{---}\mspace{14mu} h}} & (60) \\{{\pi_{z\; 2}^{TE} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{( {{T_{2}e^{\xi_{2}{({z - h})}}} + {S_{2}e^{- {\xi_{2}{({z + h})}}}}} )\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}},{\text{---}\; - h}} & (61) \\{\pi_{z\; 3}^{TE} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{T_{3}e^{\xi_{3}{({z + h})}}\lambda\;{J_{0}({\lambda\rho})}d\;{\lambda.}}}}} & (62)\end{matrix}$

Substituting equations (39) through (41) and (60) through (62) intoboundary conditions (35) and (37), we obtain the solutions for S₁, T₂,S₂ and T₃:S ₁ =[P ₂₁ξ₂ H ^(+h) +Q ₃₂ξ₃ e ^(−2ξ) ² ^(h)2ξ₂]/Δ_(v)  (63)T ₂ =[−P ₂₁ξ₁(ξ₂+ξ₃)+Q ₃₂ξ₃ e ^(−2ξ) ² ^(h)(ξ₂−ξ₁)]/Δ_(v)  (64)S ₂ =[−P ₂₁ξ₁ e ^(−2ξ) ² ^(h)(ξ₁−ξ₃)+Q ₃₂ξ₃(ξ₂+ξ₁)]/Δ_(v)  (65)T ₃ =[−P ₂₁ξ₁ e ^(−2ξ) ² ^(h)2ξ₂ −Q ₃₂ξ₂ H ^(−h)]/Δ_(v)  (66)where

$\begin{matrix}{{H^{+ h} = {\xi_{2} + \xi_{3} - {( {\xi_{2} - \xi_{3}} )e^{{- 4}\xi_{2}h}}}}{H^{- h} = {\xi_{2} + \xi_{1} - {( {\xi_{2} - \xi_{1}} )e^{{- 4}\xi_{2}h}}}}{\Delta_{v} = {{( {\xi_{2} + \xi_{1}} )( {\xi_{2} + \xi_{3}} )} - {( {\xi_{2} - \xi_{1}} )( {\xi_{2} - \xi_{3}} )e^{{- 4}\xi_{2}h}}}}{P_{21} = {\lbrack {{\frac{\beta_{2}}{\xi_{2}}e^{\xi_{2}{({z_{0} - h})}}} + Q_{2} + {P_{2}e^{{- 2}\xi_{2}h}}} \rbrack - \lbrack {{\frac{\beta_{1}}{\xi_{1}}e^{- {\xi_{1}{({z_{0} - h})}}}} + P_{1}} \rbrack}}} & (67)\end{matrix}$

For the case of the source in the middle or second layer β₂=1:

$\begin{matrix}{P_{21} = {\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}}( {k_{1}^{2} - k_{2}^{2}} )}} & (68) \\{Q_{32} = {\lbrack {{\frac{\beta_{3}}{\xi_{3}}e^{\xi_{3}{({z_{0} + h})}}} + Q_{3}} \rbrack - \lbrack {{\frac{\beta_{2}}{\xi_{2}}e^{- {\xi_{2}{({z_{0} + h})}}}} + {Q_{2}e^{{- 2}\xi_{2}h}} + P_{2}} \rbrack}} & (69)\end{matrix}$

For the case of the source in the middle or second layer β₂=1:

$\begin{matrix}{Q_{32} = {\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}}( {k_{2}^{2} - k_{3}^{2}} )}} & (70)\end{matrix}$

From equations (63) to (66) note that S₁, T₂, S₂ and T₃ split into anupper (U) part S₁ ^(U), T₂ ^(U), S₂ ^(U) and T₃ ^(U) and a lower (L)part S₁ ^(L), T₂ ^(L), S₂ ^(L) and T₃ ^(L). Consequently, π_(z) ^(TE)splits into an upper (U) part π_(z) ^(TEU) associated with the upper bedboundary at z=h and a lower (L) part π_(z) ^(TEL) associated with thelower bed boundary at z=−h such that:π_(z) ^(TE)=π_(z) ^(TEU)+π_(z) ^(TEL),  (71)

For the upper (U) part:S ₁ ^(U) =P ₂₁ξ₂ H ^(+h)/Δ_(v)  (72)T ₂ ^(U) =−P ₂₁ξ₁(ξ₂+ξ₃)/Δ_(v)  (73)S ₂ ^(U) =−P ₂₁ξ₁ e ^(−2ξ) ² ^(h)(ξ₂−ξ₃)/Δ_(v)  (74)T ₃ ^(U) =−P ₂₁ξ₁ e ^(−2ξ) ² ^(h)2ξ₂/Δ_(v)  (75)from (63) through (66).

The upper (U) part of π_(z) ^(TE) is:

$\begin{matrix}{\mspace{20mu}{\pi_{z\; 1}^{TEU} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{P_{21}}{\Delta_{v}}\xi_{2}H^{+ h}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}}} & (76) \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEU} = {\frac{- M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{P_{21}}{\Delta_{v}}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z - h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z - h + {4h}})}}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (77) \\{= {\frac{- M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{P_{21}}{\Delta_{v}}e^{{- 2}\xi_{2}h}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z + h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z + h})}}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\mspace{11mu}\lambda}}}} & (78) \\{\mspace{20mu}{\text{---} - h}} & \; \\{\mspace{20mu}{\pi_{z\; 3}^{TEU} = {\frac{- M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{P_{21}}{\Delta_{v}}e^{{- 2}\xi_{2}h}\xi_{1}2\xi_{2}e^{\xi_{3}{({z + h})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}}} & (79)\end{matrix}$

For the case of the source in the middle or second layer, β₂=1 using P₂₁(68) π_(z) ^(TEU) becomes:

$\begin{matrix}{\pi_{z\; 1}^{TEU} = {\frac{M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- \; 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}\xi_{2}H^{+ h}e^{- {\xi_{1}{({z - h})}}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\mspace{11mu}\lambda}}}} & {(6)(80)} \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEU} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{- {\xi_{2}{({z - h})}}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z - h + {4h}})}}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(7)(81)} \\{= {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}e^{{- 2}\xi_{2}h}{\xi_{1}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z + h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z + h})}}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(8)(82)} \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 3}^{TEU} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2P^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{v}\Delta_{h}}e^{{- 2}\xi_{2}h}\xi_{1}2\xi_{2}e^{- {\xi_{3}{({z + h})}}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(9)(83)}\end{matrix}$

It is the difference in material properties k²=ω²με(20) (k₁ ²−k₂ ²) atz=h that drive the π_(z) ^(TEU) part of a VMD. Similarly for π_(z)^(TEU) for a HMD.

Using P′ (52) π_(z2) ^(TEU) (7) and (81) splits into 4 termsπ_(z2) ^(TEU)=π_(z2) ^(TEU1)+π_(z2) ^(TEU2)+π_(z2) ^(TEU3)+π_(z2)^(TEU4)  (84)where, for the case of the source is in the middle or second layer β₂=1:

$\begin{matrix}{\pi_{z\; 2}^{{TEU}\; 1} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{1}( {k_{32} + k_{23}} )}( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({{+ {({z_{0} - h})}} + {({z - h})}})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (85) \\{\pi_{z\; 2}^{{TEU}\; 2} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{1}( {k_{32} + k_{23}} )}( {\xi_{2} - \xi_{3}} )e^{+ {\xi_{2}{({z_{0} - z})}}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (86) \\{\pi_{z\; 2}^{{TEU}\; 3} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{1}( {k_{32} - k_{23}} )}( {\xi_{2} + \xi_{3}} )e^{- {\xi_{2}{({z_{0} - z})}}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (87) \\{\pi_{z\; 2}^{{TEU}\; 4} = {\frac{- M_{v}}{4\pi}( {k_{1}^{2} - k_{2}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}\xi_{1}\;( {k_{32} - k_{23}} )( {\xi_{2} - \xi_{3}} )e^{\xi_{2}{({{- {({z_{0} - {({- h})}})}} - {({z - {({- h})}})}})}}e^{{- 4}\xi_{2}h} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (88)\end{matrix}$

Because of the factor e^(−4ξ) ² ^(h), π_(z2) ^(TEU2), π_(z2) ^(TEU3) andπ_(z2) ^(TEU4) are usually small. π_(z2) ^(TEU1) decreases in thedownward direction and is thus designated TE down.

In one possible embodiment, it is best to solve for π_(z) ^(TEU) andπ_(z) ^(TEL) on a bed boundary by bed boundary basis. For example, doingthe upper bed boundary at z=h for π_(z) ^(TEU). Using the form of (76)through (79) for the upper bed boundary at z=h, π_(z) ^(TEU) can beexpressed as:

$\begin{matrix}{\pi_{z\; 1}^{TEU} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{S_{1}^{U}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (89) \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEU} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{{R_{2}^{U}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z - h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z - h + {4h}})}}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (90) \\{= {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{R_{2}^{U}{e^{{- 2}\xi_{2}h}\lbrack {{( {\xi_{2} + \xi_{3}} )e^{\xi_{2}{({z + h})}}} + {( {\xi_{2} - \xi_{3}} )e^{- {\xi_{2}{({z + h})}}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (91) \\{\mspace{20mu}{\text{---} - h}} & \; \\{\pi_{z\; 3}^{TEU} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{R_{2}^{U}e^{{- 2}\xi_{2}h}2\xi_{2}e^{\xi_{3}{({z + h})}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (92)\end{matrix}$Solving for S₁ ^(U) and R₂ ^(U) using boundary conditions (35) and (37)at z=h:S ₁ ^(U) =P ₂₁ξ₂ H ^(+h)/Δ_(v)  (72)(93)R ₂ ^(U) =−P ₂₁ξ₁/Δ_(v)  (94)which produces results identical to (76) through (79).

For the lower (L) part:S ₁ ^(L) =Q ₃₂ξ₃ e ^(−2ξ) ² ^(h)2ξ₂/Δ_(v)  (95)T ₂ ^(L) =Q ₃₂ξ₃ e ^(−2ξ) ² ^(h)(ξ₂−ξ₁)/Δ_(v)  (96)S ₂ ^(L) =Q ₃₂ξ₃(ξ₂+ξ₁)/Δ_(v)  (97)T ₃ ^(L) =−Q ₃₂ξ₂ H ^(−h)/Δ_(v)  (98)from (63) through (66) The lower (L) part of π_(z) ^(TE) becomes:

$\begin{matrix}{\pi_{z\; 1}^{TEL} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{Q_{32}}{\Delta_{v}}e^{{- 2}\xi_{2}h}\xi_{3}2\xi_{2}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (99) \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEL} = {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{Q_{32}}{\Delta_{v}}e^{{- 2}\xi_{2}h}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z - h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z - h})}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} &  100 ) \\{= {\frac{M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{Q_{32}}{\Delta_{v}}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z + h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z + h - {4h}})}}}} \rbrack}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (101) \\{\mspace{20mu}{\text{---} - h}} & \; \\{= {\frac{- M_{v}}{4\pi}{\int_{0}^{\infty}{\frac{Q_{32}}{\Delta_{v}}\xi_{2}H^{- h}e^{- {\xi_{3}{({z + h})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (102)\end{matrix}$

For the case where the source is in the middle or second layer β₂=1,using Q₃₂ (70) π_(z) ^(TEL) becomes:

$\begin{matrix}{\pi_{z\; 1}^{TEL} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}e^{{- 2}\xi_{2}h}\xi_{3}2\xi_{2}e^{- {\xi_{1}{({z - h})}}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(10)(103)} \\{\mspace{20mu}{\text{---}h}} & \; \\{\pi_{z\; 2}^{TEL} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}e^{{- 2}\xi_{2}h}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z - h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z - h})}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(11)(104)} \\{= {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{\Delta_{h}\Delta_{v}}{\xi_{3}\lbrack {{( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({z + h})}}}} + {( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({z + h - {4h}})}}}} \rbrack} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(12)(105)} \\{\mspace{20mu}{{\text{---}z} = {- h}}} & \; \\{\pi_{z\; 3}^{TEL} = {\frac{- M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2Q^{\prime}e^{{- 2}\xi_{2}h}}{{\Delta_{h}\Delta_{v}}\;}\xi_{2}H^{- h}e^{\xi_{3}{({z + h})}} \times \lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & {(13)(106)}\end{matrix}$

It is the difference in material properties k²=ω²με (20) (k₂ ²−k₃ ²)between layer 2 and layer 3 at z=−h that drives the π_(z) ^(TEL) part ofa VMD. Similarly for π_(z) ^(TEL) for a HMD.

Using Q′ (equation (53)) π_(z2) ^(TEL) (see equations (12) (105)) splitsinto 4 termsπ_(z2) ^(TEL)=π_(z2) ^(TEL1)+π_(z2) ^(TEL2)+π_(z2) ^(TEL3)+π_(z2)^(TEL4)  (104)

where, for the case of the source is in the middle or second layer β₂=1:

$\begin{matrix}{\pi_{z\; 2}^{{TEL}\; 1} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{3}( {k_{12} + k_{21}} )}( {\xi_{2} + \xi_{1}} )e^{- {\xi_{2}{({{({z_{0} - {({- h})}})} + {({z - {({- h})}})}})}}}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (108) \\{\pi_{z\; 2}^{{TEL}\; 2} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{3}( {k_{12} + k_{21}} )}( {\xi_{2} - \xi_{1}} )e^{- {\xi_{2}{({z_{0} - z})}}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (109) \\{\pi_{z\; 2}^{{TEL}\; 3} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{3}( {k_{12} - k_{21}} )}( {\xi_{2} + \xi_{1}} )e^{\xi_{2}{({z_{0} - z})}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}({\lambda\rho})}d\;\lambda}}}} & (110) \\{\pi_{z\; 2}^{{TEL}\; 4} = {\frac{M_{v}}{4\pi}( {k_{2}^{2} - k_{3}^{2}} ){\int_{0}^{\infty}{\frac{2}{\Delta_{v}\Delta_{h}}{\xi_{3}( {k_{12} - k_{21}} )}( {\xi_{2} - \xi_{1}} )e^{\xi_{2}{({{({z_{0} - h})} + {({z - h})}})}}e^{{- 4}\xi_{2}h}\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}} & (111)\end{matrix}$

Because of the factor e^(−4ξ) ² ^(h), π_(z2) ^(TEL2), π_(z2) ^(TEL3) andπ_(z2) ^(TEL4) are usually small. π_(z2) ^(TEL1) decreases in the upwarddirection and is thus designated TE up.

Electromagnetic Fields Due to a Horizontal Magnetic Dipole

For a horizontal magnetic dipole, it is convenient to use both therectangular and the cylindrical coordinates with the magnetic dipolelocated at x=0, y=0, or ρ=0 and z=z₀, pointing in the x direction, asshown in FIG. 14. Although equation (19) might indicate that only π_(x)^(TM) exists for this case, which is not true. In fact, the Hertzpotential has a z component as well as an x componentπ=π_(x) ^(TM) {circumflex over (x)}+π _(z) ^(TE) {circumflex over(z)}.  (112)The tangential components of the fields are related to the Hertzpotential as:

$\begin{matrix}{{E_{x} = {i\;\omega\;\mu\;\frac{d\;\pi_{z}^{TE}}{d\; y}}},{and}} & (113) \\{{E_{y} = {i\;\omega\;{\mu( {\frac{d\;\pi_{x}^{TM}}{d\; z} - \frac{d\;\pi_{z}^{TE}}{d\; x}} )}}},} & (114)\end{matrix}$from (17) and

$\begin{matrix}{{H_{y} = {\frac{d}{d\; y}( {\nabla{\cdot \pi}} )}},\mspace{14mu}{and}} & (115) \\{H_{x} = {{k^{2}\pi_{x}^{TM}} + {\frac{d}{d\; x}( {\nabla{\cdot \pi}} )}}} & (116)\end{matrix}$from (18).

To ensure continuity of the tangential E and H field components acrossthe bed boundaries at z=±h, the following boundary conditions on theHertz potential must be satisfied, assuming μ₁=μ₂=μ₃=μ:

from (113)π_(zj) ^(TE)=π_(z(j+1)) ^(TE),  (117)

from (114) and (117)

$\begin{matrix}{\frac{d\;\pi_{xj}^{TM}}{d\; z} = \frac{d\;\pi_{x{({j + 1})}}^{TM}}{d\; z}} & (118)\end{matrix}$

from (115)∇·π_(j)=∇·π_(j+1),  (119)and

from (116) and (119)k _(j) ²π_(xj) ^(TM) =k _(j+1) ²π_(x(j+1)) ^(TM).  (120)

The Hertz potential π_(x) ^(TM) satisfies the inhomogeneous partialdifferential equation (19) with M_(s)=M_(h)δ(r−z₀{circumflex over(z)}){circumflex over (x)}. Accordingly, π_(x) ^(TM) can be expressedas:

$\begin{matrix}{\mspace{20mu}{{\pi_{x\; 1}^{TM} = {\frac{M_{h}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{1}}{\xi_{1}}e^{{- \xi_{1}}{{z - z_{0}}}}} + {P_{1}e^{- {\xi_{1}{({z - h})}}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}},}} & (121) \\{\mspace{20mu}{\text{---}h}} & \; \\{{\pi_{x\; 2}^{TM} = {\frac{M_{h}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{2}}{\xi_{2}}{e\;}^{{- \xi_{2}}{{z - z_{0}}}}} + {Q_{2}e^{\xi_{2}{({z - h})}}} + {P_{2}e^{- {\xi_{2}{({z + h})}}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;\lambda}}}},} & (122) \\{\mspace{20mu}{\text{---} - h}} & \; \\{\mspace{20mu}{\pi_{x\; 3}^{TM} = {\frac{M_{h}}{4\pi}{\int_{0}^{\infty}{\lbrack {{\frac{\beta_{3}}{\xi_{3}}e^{{- \xi_{3}}{{z - z_{0}}}}} + {Q_{3}e^{\xi_{3}{({z + h})}}}} \rbrack\lambda\;{J_{0}( {\lambda\;\rho} )}d\;{\lambda.}}}}}} & (123)\end{matrix}$P₁, Q₂, P₂, Q₃ (42) through (45) are from boundary conditions (120) and(118).

π_(x) ^(TM) alone cannot satisfy the boundary condition on ∇·π, (119).This boundary condition may be thought of as a coupling mechanismbetween π_(x) ^(TM) and π_(z) ^(TE) (112). π_(z) ^(TE) in each of thethree layers is:

$\begin{matrix}{{\pi_{z\; 1}^{TE} = {\frac{M_{h}}{4\pi}\cos\;\phi{\int_{0}^{\infty}{S_{1}e^{- {\xi_{1}{({z - h})}}}\lambda\;{J_{1}( {\lambda\;\rho} )}d\;\lambda}}}},} & (124) \\{\text{---}h} & \; \\{{\pi_{z\; 2}^{TE} = {\frac{M_{h}}{4\pi}\cos\;\phi{\int_{0}^{\infty}{( {{T_{2}e^{\xi_{2}{({z - h})}}} + {S_{2}e^{- {\xi_{2}{({z + h})}}}}} )\lambda\;{J_{1}( {\lambda\;\rho} )}d\;\lambda}}}},} & (125) \\{\text{---} - h} & \; \\{\pi_{z\; 3}^{TE} = {\frac{M_{h}}{4\pi}\cos\;\phi{\int{T_{3}e^{\xi_{3}{({z + h})}}\lambda\;{J_{1}( {\lambda\;\rho} )}d\;{\lambda.}}}}} & (126)\end{matrix}$

Substituting equations (121) through (123) and (124) through (126) intoboundary conditions (117) and (119), we obtain the solutions for S₁, T₂,S₂ and T₃S ₁ =[P ₂₁ B ^(+h) +Q ₃₂ e ^(−2ξ) ² ^(h)2ξ₂]λ/Δ_(v)  (127)T ₂ =[P ₂₁(ξ₂+ξ₃)+Q ₃₂ e ^(−2ξ) ² ^(h)(ξ₂−ξ₁)]λ/Δ_(v)  (128)S ₂ =[P ₂₁ e ^(−2ξ) ² ^(h)(ξ₂−ξ₃)+Q ₃₂(ξ₂+ξ₁)]λ/Δ_(v)  (129)T ₃ =[P ₂₁ e ^(−2ξ) ² ^(h)2ξ₂ +Q ₃₂ B ^(−h)]λ/Δ_(v)  (130)whereB ^(+h)=ξ₂+ξ₃+(ξ₂−ξ₃)e ^(−4ξ) ² ^(h)  (131)andB ^(−h)=ξ₂+ξ₁+(ξ₂−ξ₁)e ^(−4ξ) ² ^(h).  (132)

From equations (127) to (130), it is noted that S₁, T₂, S₂ and T₃ splitinto an upper (U) part S₁ ^(U), T₂ ^(U), S₂ ^(U) and T₃ ^(U) and a lower(L) part S₁ ^(L), T₂ ^(L), S₂ ^(L) and T₃ ^(L). Consequently, π_(z)^(TE) splits into an upper (U) part π_(z) ^(TEU) associated with theupper bed boundary at z=11 and a lower (L) part π_(z) ^(TEL) associatedwith the lower bed boundary at z=−h such that:π_(z) ^(TE)=π_(z) ^(TEU)+π_(z) ^(TEL).  (133)Very similar to (71) for a VMD.

The boundary conditions for a Vertical Electric Dipole are (SeeSommerfeld 1949 hereinbefore):

$\begin{matrix}{{k_{\; j}^{2}\pi_{zj}} = {k_{({j + 1})}^{2}\pi_{z{({j + 1})}}}} & (134) \\{\frac{d\;\pi_{zj}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}}{d\; z}} & (135)\end{matrix}$

The above boundary conditions can be written as:

$\begin{matrix}{\pi_{zj}^{TE} = \pi_{z{({j + 1})}}^{TE}} & (136) \\{\frac{d\;\pi_{zj}^{TE}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TE}}{d\; z}} & (137) \\{{k_{j}^{2}( {\pi_{j}^{TE} + \pi_{j}^{TM}} )} = {k_{({j + 1})}^{2}( {\pi_{({j + 1})}^{TE} + \pi_{({j + 1})}^{TM}} )}} & (138) \\{\frac{d\;\pi_{zj}^{TM}}{d\; z} = \frac{d\;\pi_{z{({j + 1})}}^{TM}}{d\; z}} & (139)\end{matrix}$

(138) is equivalent to (134) and (137) plus (139) is equivalent to (135)using π_(z)=π_(z) ^(TE)+π_(z) ^(TM).

For completeness, the boundary conditions for a Horizontal ElectricDipole are:

$\begin{matrix}{{k_{j}^{2}\pi_{x{(j)}}^{TE}} = {k_{({j + 1})}^{2}\pi_{x|{({j + 1})}}^{TE}}} & (140) \\{{k_{j}^{2}\frac{d\;\pi_{xj}^{TE}}{d\; z}} = {k_{({j + 1})}^{2}\frac{d\;\pi_{x{({j + 1})}}^{TE}}{d\; z}}} & (141) \\{{k_{j}^{2}\pi_{z{(j)}}^{TM}} = {k_{({j + 1})}^{2}\pi_{z|{({j + 1})}}^{TM}}} & (142) \\{{\nabla{\cdot \pi}} = {\nabla{\cdot \pi}}} & (143)\end{matrix}$

While the invention has been described in terms of components of variousmethods, individual components or various groups of components of themethods described hereinbefore can be utilized. For example, in oneembodiment, it may be useful to utilize transverse magnetic term one byitself. Alternatively, it may be useful to determine selected terms forparticular boundaries. Other examples are provided above and in theclaims. The invention can be implemented by first determining whathappens at the boundaries. Alternatively, the invention may firstcalculate a log for each layer. Combinations of these approaches mayalso be utilized.

While the invention has been described in terms of methods, it will beappreciated that the methods may be utilized within devices, whereby theinvention also describes physical devices. Thus, the present inventionmay be embodied as a machine or system for producing logs. Raw ortransformed electrical signals detected by the system are transformed todescribe material properties of beds, bed boundary orientations andpositions, which may not be apparent or may be inaccurate when simplyviewing the electrical signals. It is well known that materialproperties of beds, bed boundary orientations and positions often haveinaccuracies at the bed boundaries. For example, instead of a claim to amethod, the present invention might also be described and claimed,referring to claims hereinafter, as a system making a log of materialproperties in a plurality of beds from an instrument which produces anelectromagnetic field, wherein the system comprises one or moreelectronic components programmed for estimating material properties forsaid plurality of beds, estimating positions for a plurality of bedboundaries, estimating orientations for said plurality of bed boundarieswherein said bed boundary orientations are individually variable, andutilizing said positions, said orientations, and said materialproperties to compute said log.

As well, the invention may comprise software which may be stored on astorage medium and utilized in a computer memory, and/or implemented asa series of instructions, depending on the programming languageutilized. For example, the invention may be implemented in Fortran ormany other suitable computer languages.

As discussed above, it is noted once again that transverse electric downor transverse electric up is not the same as transverse electric termsone, two, three or four.

While the present invention is described in terms of a geologicallayered environment and electromagnetic tools, the invention may also beutilized for other purposes, e.g., medical purposes such as acousticanalysis of a human body, seismic analysis, or other tools and layeredenvironments. The wave equations are useful for acoustic wave analysis,utilizing higher and/or lower frequencies, and the like within otherlayered environments.

Accordingly, the foregoing disclosure and description of the inventionis illustrative and explanatory thereof, and it will be appreciated bythose skilled in the art, that various changes in the ordering of steps,ranges, and/or attributes and parameters related to the steps and/ormaterials, as well as in the details of the illustrations orcombinations of features of the methods discussed herein, may be madewithout departing from the spirit of the invention. Thus, while theinvention has been described in connection with a preferred embodiment,it is not intended to limit the scope of the invention to the particularform set forth, but on the contrary, it is intended to cover suchalternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention.

What is claimed is:
 1. A method, comprising: receiving data, producedfrom at least one instrument, representing a plurality of beds in ageologic formation relative to position in a well bore; estimating, fromthe data, positions within the geologic formation of two bed boundariestraversed by the well bore; estimating, from the data, orientations ofthe two bed boundaries traversed by the well bore, wherein saidestimated orientations are free to be non-parallel relative to oneanother, and wherein estimating the orientations comprises using aprocessor to, for each given bed boundary of the two bed boundaries,generate a model representing expected change in electromagneticproperties to be produced by the at least one instrument as it traversesthe given bed boundary, wherein the model is dependent on apparentorientation of the given bed boundary such that the apparent orientationchanges as the model changes, and fit the model to the data by changingan initial apparent orientation of the given bed boundary, including atleast one of dipping angle or direction of dip, until the modelapproximates a portion of the data corresponding to estimated positionfor the given bed boundary, to thereby obtain a fitted model; andcomputing a physical property of a given bed of the plurality of beds inthe geologic formation, wherein the given bed is between the two bedboundaries, in a manner dependent on the fitted models; wherein thephysical property comprises at least one of conductivity or resistivityof the given bed.
 2. The method of claim 1, wherein the physicalproperty is conductivity of the given bed.
 3. The method of claim 1,wherein the physical property is resistivity of the given bed.
 4. Themethod of claim 1, wherein receiving data includes receiving a measuredlog that varies as a function of position within the well bore, andwherein estimating the positions comprises: identifying positions withinthe well bore corresponding to variations in the measured log; andestimating the positions of the two bed boundaries dependent on thepositions within the well bore where the variations are experienced. 5.The method of claim 1, wherein: the method further comprises, for agiven one of the bed boundaries, identifying a measure of a verticalmagnetic dipole (VMD) response and a measure of a horizontal magneticdipole (HMD) response; adjusting the model in a manner which simulatesvariation in the apparent orientation comprises combining the measure ofthe VMD response with the measure of the HMD response to obtain acombined result; and using the at least one processor to fit the modelto the data further comprises simulating variation in the apparentorientation for the given bed boundary by varying relative contributionof the measure of the VMD response and the measure of the HMD responsein the combined result, and fitting the combined result to the measuredlog to obtain the fitted model for the given bed boundary, and selectinga specific apparent orientation dependent on the relative contributionof the measure of the VMD response and the measure of the HMD responseassociated with the fitted model for the given bed boundary.
 6. Themethod of claim 1, wherein: the method further comprises, for at leastone of the two bed boundaries, identifying a vertical magnetic dipole(VMD) measure and a horizontal magnetic dipole (HMD) measure; the modelis dependent on a variable combination of the VMD measure and the HMDmeasure; and using the at least one processor to fit the model to thedata comprises adjusting the variable combination of the VMD measure andthe HMD measure, in a manner representing variation of apparentorientation.
 7. The method of claim 1, wherein using the at least oneprocessor to fit the model to the data comprises: adjusting at least oneof conductivity or resistivity of a bed adjacent to the given bedboundary.
 8. The method of claim 1, wherein the data iscomputer-readable and receiving the data comprises receiving the dataelectronically.
 9. The method of claim 5, wherein combining comprisessumming the measure of the VMD response with the measure of the HMDresponse to obtain the combined result.
 10. The method of claim 5,wherein the measure of the VMD response comprises a transverse electriccomponent of the VMD response and the measure of the HMD responsecomprises a transverse electric component of the HMD response.
 11. Themethod of claim 5, wherein the measure of the VMD response comprises atransverse magnetic component of the VMD response and the measure of theHMD response comprises a transverse magnetic component of the HMDresponse.
 12. The method of claim 7, wherein: adjusting the at least oneof conductivity or resistivity further comprises adjusting the at leastone of conductivity or resistivity for each of two beds adjacent thegiven bed boundary.
 13. The method of claim 8, wherein the instrument isan induction sonde and wherein the data represents a measured logrepresenting at least one of conductivity or resistivity as a functionof well depth.
 14. An apparatus comprising: means for receiving data,produced from at least one instrument, representing a plurality of bedsin a geologic formation relative to position in a well bore; means forestimating, from the data, positions within the geologic formation oftwo bed boundaries traversed by the well bore; means for estimating,from the data, orientations of the two bed boundaries traversed by thewell bore, wherein said estimated orientations are free to benon-parallel relative to one another, and wherein said means forestimating comprises means for, for each given bed boundary of the twobed boundaries, generating a model representing expected change inelectromagnetic properties to be produced by the at least one instrumentas it traverses the given bed boundary, wherein the model is dependenton apparent orientation of the given bed boundary such that the apparentorientation changes as the model changes, and fitting the model to thedata by changing an initial apparent orientation of the given bedboundary, including at least one of dipping angle or direction of dip,until the model approximates a portion of the data corresponding toestimated position for the given bed boundary, to thereby obtain afitted model; and means for computing a physical property of a given bedof the plurality of beds in the geologic formation, wherein the givenbed is between the two bed boundaries, in a manner dependent on thefitted models; wherein the physical property comprises at least one ofconductivity or resistivity of the given bed.
 15. An apparatuscomprising instructions stored on non-transitory machine-readable media,the instructions when executed to cause at least one processor to:receive data, produced from at least one instrument, representing aplurality of beds in a geologic formation relative to position in a wellbore; estimate, from the data, positions within the geologic formationof two bed boundaries traversed by the well bore; estimate, from thedata, orientations of the two bed boundaries traversed by the well bore,wherein said estimated orientations are free to be non-parallel relativeto one another, wherein the instruction are to further cause the atleast one processor to, for each given bed boundary of the two bedboundaries, generate a model representing expected change inelectromagnetic properties to be produced by the at least one instrumentas it traverses the given bed boundary, wherein the model is dependenton apparent orientation of the given bed boundary such that the apparentorientation changes as the model changes, and fit the model to the databy changing an initial apparent orientation of the given bed boundary,including at least one of dipping angle or direction of dip, until themodel approximates a portion of the data corresponding to estimatedposition for the given bed boundary, to thereby obtain a fitted model;and compute a physical property of a given bed of the plurality of bedsin the geologic formation, wherein the given bed is between the two bedboundaries, in a manner dependent on the fitted models; wherein thephysical property comprises at least one of conductivity or resistivity.16. The apparatus of claim 15, wherein the physical property isconductivity of the given bed.
 17. The apparatus of claim 15, whereinthe physical property is resistivity of the given bed.
 18. The apparatusof claim 15, wherein the instructions when executed are to cause the atleast one processor to receive a measured log that varies as a functionof position within the well bore, and to: identify positions within thewell bore where variations in the measured log are experienced; andestimate the positions of the two bed boundaries dependent on thepositions where the variations in the measured log are experienced. 19.The apparatus of claim 15, wherein the instructions when executed are tocause the at least one processor to: identify a measure of a verticalmagnetic dipole (VMD) response and a measure of a horizontal magneticdipole (HMD) response; combine the measure of the VMD response with themeasure of the HMD response to obtain a combined result; simulatevariation in the apparent orientation by varying relative contributionof the measure of the VMD response and the measure of the HMD responsein the combined result, and fitting the combined result to the measuredlog to obtain the fitted model for the given bed boundary; and select aspecific apparent orientation for the given bed boundary dependent onthe relative contribution of the measure of the VMD response and themeasure of the HMD response associated with the fitted model for thegiven bed boundary.
 20. The apparatus of claim 15, wherein: theinstructions when executed are to cause the at least one processor to,for at least one of the two bed boundaries, identify a vertical magneticdipole (VMD) measure and a horizontal magnetic dipole (HMD) measure; themodel is dependent on a variable combination of the VMD measure and theHMD measure; and the instructions when executed are to cause the atleast one processor to, for at least one of the two bed boundaries, fitthe model to the data by adjusting the variable combination of the VMDmeasure and the HMD measure, in a manner representing variation of theapparent orientation.
 21. The apparatus of claim 15, wherein theinstructions when executed are to cause the at least one processor tofit the model to the data by: adjusting at least one of conductivity orresistivity of a bed adjacent to the given bed boundary.
 22. Theapparatus of claim 15, wherein the data is computer-readable and theinstructions when executed are to cause the at least one processor toreceive the data electronically.
 23. The apparatus of claim 19, whereinthe instructions when executed are to cause the at least one processorto sum the measure of the VMD response with the measure of the HMDresponse to obtain the combined result.
 24. The apparatus of claim 19,wherein the measure of the VMD response comprises a transverse electriccomponent of the VMD response and the measure of the HMD responsecomprises a transverse electric component of the HMD response.
 25. Theapparatus of claim 19, wherein the measure of the VMD response comprisesa transverse magnetic component of the VMD response and the measure ofthe HMD response comprises a transverse magnetic component of the HMDresponse.
 26. The apparatus of claim 21, wherein the instructions whenexecuted are to cause the at least one processor to: adjust at least oneof conductivity or resistivity for each of two beds adjacent the givenbed boundary.
 27. The apparatus of claim 22, wherein the instrument isan induction sonde and wherein the data represents a measured logrepresenting at least one of conductivity or resistivity as a functionof well depth.